The invention relates to a process for the separation of two liquids by passing a dispersion of negatively charged droplets of a liquid in an aqueous liquid along the surface of a solid material, thus forming coalesced dispersed liquid and, after the aqueous liquid has left the solid material, separating coalesced dispersed liquid from the aqueous liquid. The invention also relates to a process for the manufacture of such a solid material. The invention further relates to an apparatus for said separation and containing said solid material.
In the industry use is frequently made of a process for the above mentioned separation, for example, of oil from a dispersion thereof in an aqueous liquid. Usually, these oil droplets are negatively charged. Such an oil may be a hydrocarbon oil, for example, crude mineral oil, gas oil or kerosene, or a vegetable oil, for example, soya bean or cotton seed oil. Examples of such dispersions are oil-contaminated cooling water, tanker washings containing crude mineral oil and water used for product treatment in oil refineries. Before such used water can be released to rivers, canals or the sea, it is necessary to remove dispersed oil therefrom.
By passing said dispersion along the surface of a suitable solid material the dispersed droplets tend to coalesce, forming larger drops, and these larger drops can more easily be removed from the aqueous liquid by, for example, gravity separation. However, it has sometimes not been found easy to obtain a satisfactory coalescence efficiency. The coalescence efficiency, expressed in a percentage, is defined herein as: ##EQU1## in which F is the dispersed amount of liquid in the aqueous liquid upstream of the surface of the solid material and P is the dispersed amount of liquid in the aqueous liquid downstream of the surface of the solid material, after removal of the coalesced dispersed liquid.
An example of a process having an efficiency susceptible of improvement is described in British patent specification No. 2,038,300. By this known process the average dispersed amount of oil was reduced from 33 mg/l to 9 mg/l, which represents an efficiency of only 73%. The Applicants have found that during longer term continuous operation the bed progressively becomes blocked with liquid originating from the dispersed liquid, so that the operation has to be halted. In this known process the fibers are excessively wetted by liquid originating from the dispersed liquid, act primarily as a filter and poorly release coalesced liquid originating from the dispersed liquid; in such a situation a high backpressure is observed. The backpressure is defined as the difference between the pressure upstream of the coalescer bed and that downstream of the coalescer bed. Moreover, the solid material being used in this known process is manufactured in two steps and each of these steps is fairly complicated. In the first step, an inorganic solid containing surface hydroxyl groups is reacted with certain silane coupling agents, and in the second step the product formed in the first step is reacted at elevated temperature with an N-substituted imide of an alkenyl substituted dicarboxylic acid. Both steps require long reaction times--a number of hours--and are carried out in hydrocarbon solvents. Furthermore, it is recommended that said alkenyl groups be long, having a molecular weight in the range 300 to 2100.
A process has now been found by which the dispersed negatively charged droplets can be removed with a considerably improved coalescence efficiency which can be retained for a very long time. For this purpose solid material is used which can be prepared in a considerably simpler manner.